1. Field of the Invention
The present invention relates generally to the production of butanol by fermentation, and specifically to single vessel butanol production via generation of a micro-environment to maintain coincident subpopulations of acidogenic- and solventogenic-phase cells.
2. Background Information
Butanol (n-butanol or n-butyl alcohol) may be produced by the fermentation of carbohydrates which break down into products such as sugars containing five and six carbon atoms (e.g., glucose). This process was developed by Charles Weizmann during World War I (see, e.g., U.S. Pat. Nos. 1,315,585; 1,329,214; 1,437,697). Briefly, the Weizmann process comprises fermentation of a suitable feedstock in the presence of Clostridium acetobutylicum which convert the feedstock into a solvent mixture of acetone, butanol, and ethanol (ABE). In the solvent mixture, the ratio of butanol to acetone to ethanol is usually 6:3:1.
ABE fermentation is a biphasic process: during the first (acidogenic) phase, logarithmic growth is accompanied by acetic and butyric acid production which also causes a concomitant and necessary drop in pH. In the second (solventogenic) phase, growth ceases and the solvents are produced with the simultaneous consumption of the aforementioned acids, including the further consumption of the input feedstock. Hydrogen and carbon dioxide are continuously produced throughout the fermentation.
The resulting solvents are produced in dilute aqueous solutions, generally from about 1% to 2% by weight, so that their recovery in pure form involves their separation from large amounts of water. The expense of separation has been such that production of these chemicals by fermentation has not been able to compete with solvent production from petroleum-based sources. However, the concern for global warming, a desire to achieve energy independence, and the increase in prices of petrochemical-derived feedstocks have led to the revival of interest in processes which can convert renewable, unrefined materials into simple organic chemicals.
One problem associated with ABE fermentation by Clostridium microorganisms is butanol toxicity to the culture (i.e., the maximum tolerated butanol concentration is approximately 2%, irrespective of sugar concentration). Toxicity may be avoided by continuous removal of butanol during the process for maximum production of solvents. Various butanol removal systems, including adsorption, pervaporation, perstraction, reverse osmosis, liquid-liquid extraction, and gas stripping, have met with limited success. Further, all of the above methods increase production costs. Thus, there remains a need in the art for an improved process to generate solvents from these organisms.